(1) Field of the Invention
The present invention relates in general to the fabrication of semiconductor devices for integrated circuits, and more particularly to a method for forming a trench isolation having conformal top and bottom round trench liner oxides. This provides improved gap filling with chemical vapor deposited (CVD) oxides and the rounded top and bottom corners reduce the junction leakage for devices, such as field effect transistors (FETs).
(2) Description of the Prior Art
In fabricating semiconductor circuits, field oxide (FOX) regions are formed in and on the silicon substrate to surround and electrically isolate the device areas. One of the most common and cost-effective methods of forming this field oxide isolation in the semiconductor industry is by the LOCal Oxidation of Silicon (LOCOS) technique. In the LOCOS technique a patterned silicon nitride layer is used on the silicon substrate as a barrier mask to oxidation. The substrate is then oxidized to form the FOX regions. However, the LOCOS technique has several problems as the device feature sizes decrease and the circuit density increases. One problem is that the lateral oxidation of the silicon substrate under the silicon nitride mask forms what are commonly referred to in the industry as "birds+ beaks."These birds' beaks extend into and reduce the active device areas. This limits the device packing density on the chip. As minimum feature sizes on the substrate continue to decrease, it will become increasingly difficult to increase the integrated circuit density.
One method of overcoming these LOCOS limitations is to use trench isolation techniques. In this method trenches less than 0.5 micrometers (um) in width are etched in the substrate while the silicon nitride (Si.sub.3 N.sub.4) is left over the device areas. A sacrificial oxide is thermally grown to remove any substrate damage due to the plasma etching of the trenches, and a relatively thin liner oxide is grown to provide a good interface. The trenches are then filled by depositing a silicon oxide (SiO.sub.2), such as by subatmospheric chemical vapor deposition (SA-CVD). The SACVD oxide is then etched back or chemical/mechanically polished back to the Si.sub.3 N.sub.4 layer to complete the trench isolation. Unfortunately, the trench isolation has several limitations. For example, the sharp corners at the bottom of the trenches can result in high stress that can cause lattice defects in the single-crystal silicon substrate during liner oxidation and gap filling. Another problem is that sharp corners at the top of the trench can result in junction leakage currents, more specifically, can result in unwanted increases in the subthreshold currents in the channel regions along the edge of the device areas when the FETs are switched on. The device threshold voltage can also be lowered. Therefore it is desirable to form trenches having rounded top corners to increase the radius of curvature and decrease the electric field at the top corners.
One method of forming the liner oxide is to thermally oxidize the trench using a high flow rate of O.sub.2 and a relatively low temperature of 920.degree. degrees centigrade (.degree.C.) followed by a second high flow rate of O.sub.2 at 920.degree. C. Although this provides rounded top and bottom corners, the profile is not conformal around the trench wall and a thicker overhead profile makes it difficult to SA-CVD oxide gap filling of the trenches. Another approach is to form the liner oxide using a wet liner oxidation at even lower temperatures of about 850.degree. C. and at high O.sub.2 flow rates. Although the process eliminates the overhead profile and provides rounded bottom corners, the top corners are sharp and there is a thicker oxide on the trench sidewalls.
Other methods of forming trench isolation have been described in the literature. For example, Koike et al. in U.S. Pat. No. 5,578,518, describes a method for forming rounded top corners in the trench. The method involves recessing a thermally grown oxide around the top edge of the trench to expose the top corners before grown a liner oxide. Then the liner oxide is grown resulting in more rounded top corners. Another method for forming trenches with rounded corners is described by Horioka et al. in U.S. Pat. No. 5,258,332 in which the oxide mask at the edge of the trench is recessed in a hydrofluoric acid etch. Then the exposed silicon edges of the trench are subjected to a CF.sub.4 /O.sub.2 in a down-flow type etching apparatus, commonly referred to as a chemical dry etching (CDE) apparatus to round the top corners of the silicon trench. Crotti et al., U.S. Pat. No. 5,068,202, teach a method for forming rounded trenches for implanting in the sidewalls without reverting to special implant techniques. The trenches are subjected to a CF.sub.4 /O.sub.2 plasma etching in a down-stream etcher to round the trench.
However, none of the above references addresses the concerns of growing a conformal liner oxide in a sub-micron trench, while providing both rounded top and bottom corners. Therefore there is still a need for an improved liner oxide process.